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d) When CO2 is added to seawater it reacts with hydrogen and forms carbonic acid, which could cause the pH to drop. Carbonic acid, however, then immediately forms carbonate and bicarbonate, which in turn join with calcium and form calcium carbonate. This "uses up" the carbonic acid and the pH remains stable. This reaction can move both ways, depending on availability of CO2 and calcium, and this maintains seawater pH at about 8.2. This is the seawater buffer system. If more CO2 is added, and additional calcium is not available, then carbonic acid increases and pH drops, thus at first the pH stays the same and then if excess CO2 accumulates, or if calcium is depleted, it then drops.

The answer to this weeks trivia question pretty much explains how a calcium reactor works. The calcium reactor container is filled with media and saltwater. The media is made from coral rock fragments. When the CO2 is added to the calcium reactor container, an acidic solution is formed. This solution dissolves the coral rock fragments. Below is a quick video that goes into better detail on calcium reactors.